Numerous techniques have been used and proposed for measuring physiological rates. U.S. Pat. No. 4,066,069 relates to a system for detecting differences between a pre-recorded, normal, reference pulse wave obtained from an electrocardiogram (ECG) and a measured ECG pulse wave from the same subject. A phase-lock loop (PLL) is used to synchronize the rate of the reference ECG wave with the rate of the measured ECG wave to obtain a comparison between them. Specifically, in this patented system a reference heart beat signal is recirculated for comparison with an actual heart beat signal. First and second voltages are produced having amplitudes respectively representing the rate of the reference heart beat and the rate of the actual heart beat. A voltage-controlled oscillator (VCO) produces a signal output having a frequency dependent upon the difference between the first and second voltages. This VCO output signal is used to synchronize the reference heart beat signal with the actual heart beat signal.
U.S. Pat. No. 4,202,340 shows a circuit for monitoring heart activity. The circuit utilizes a sensed cardiac parameter indicative of the operation of the heart being monitored and generates a probability density function from the sensed parameter. The probability density function is analyzed and its shape is compared with the shape of a probability density function indicative of normal cardiac function. The differences in shape are used to indicate abnormal cardiac function of the heart being monitored.
U.S. Pat. No. 4,063,551 is concerned with sensing blood pulses by emitting an infrared light toward a blood vessel and monitoring the relative absorbtion of the light in the blood stream to indicate the timing of blood pulses.